Superalloy mortar tube

ABSTRACT

A finless mortar tube is made of a superalloy. The superalloy is based on one of cobalt, iron and nickel. The finless mortar tube has an integrally formed blast attenuation device. The mortar tube may be 60, 81 or 120 mm. The mortar tube is capable of a substantial increase in the rate of fire compared to conventional mortar tubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S. provisional patent application 60/522,510 filed on Oct. 7, 2004 and 60/522,566 filed on Oct. 14, 2004, which applications are hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes.

BACKGROUND OF THE INVENTION

The invention relates in general to mortar tubes, and in particular to mortar tubes without cooling fins and having reduced wall thicknesses.

Mortars tubes presently used by the United States armed forces are generally available in three sizes of nominal inside diameter, namely, 60 mm (millimeter), 81 mm and 120 mm. The current 60 mm and 81 mm mortar tubes have cooling fins that function to reduce the tube temperature during firing. The mortar tube cooling fins are expensive to manufacture and add additional weight to the mortar tube. The 120 mm mortar tube does not have cooling fins because its required rate of fire is less than the 60 mm and 81 mm mortars.

Generally speaking, the soldier in the field benefits whenever anything he/she must handle is made to weigh less. The present invention provides reduced weight mortar tubes by eliminating the need for fins on the 60 mm and 81 mm tubes and by reducing the wall thickness of all three sizes of mortar tubes.

SUMMARY OF THE INVENTION

In broad terms, the present invention is a mortar tube made of a superalloy. The superalloy may be, for example, one of cobalt based, iron based and nickel based. Because of the excellent heat strength of superalloys, the mortar tube may have an external surface that is free of cooling fins.

Another aspect of the invention is a method of making a mortar tube comprising providing a superalloy material and forming the mortar tube from the superalloy material.

The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals.

FIG. 1A is a side view of a known mortar tube.

FIG. 1B is a sectional view taken along the line 1B-1B of FIG. 1A.

FIG. 2A is a side view of a mortar tube according to the invention.

FIG. 2B is a sectional view taken along the line 2B-2B of FIG. 2A.

FIG. 3 is a graph of tube temperature vs. axial position for two finless tubes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention eliminates conventional cooling fins and reduces the wall thickness of mortar tubes by constructing the mortar tubes using a high strength superalloy. The superalloys are known and typically fall into one of three types, iron based, cobalt based and nickel based. In general, the superalloys have material strengths greater than 140 ksi at tube temperatures greater than 1000 degrees Fahrenheit. The use of a higher strength material permits a thinner wall thickness, as compared to conventional tubes.

FIG. 1A is a side view of a known 81 mm mortar tube 10. Tube 10 includes cooling fins 12 on the rear portion near the breech. The cooling fins 12 reduce the temperature of the mortar tube 10 from about 1160° F. to 1022° F. at presently required maximum rates of fire, i.e., 30 rounds per minute for 2 minutes and 15 rounds per minute sustained. These rates of fire are based on mortar ammunition having maximum design pressures of 15,800 psi. A separate blast attenuation device (BAD) 14 is attached at the muzzle end of the tube 10.

FIG. 1B is a sectional view of the tube 10 taken along the line 1B-1B of FIG. 1A. As seen in FIG. 1B, tube 10 has a wall thickness g. The steel used to make tube 10 cannot withstand the design ammo pressure loads if the tube temperature increases above 1160° F., as it would if the tube 10 had no cooling fins 12.

FIG. 2A is a side view of a mortar tube 20 according to the invention. FIG. 2B is a sectional view of the tube 20 taken along the line 2B-2B of FIG. 2A. As seen in FIG. 2B, tube 20 has a wall thickness h. Tube 20 does not have cooling fins, in particular, the rear portion 24 of tube 20, where fins would normally be formed, is without fins. A BAD 22 is formed integrally with the tube 20. Tube 20 may be formed by machining or a metal flow-forming process. Tube 20 is made of a superalloy that is one of nickel based, iron based or cobalt based.

The rate of fire (ROF) in number of rounds per minute (rds/min) for ammunition dictates the temperatures that a mortar tube will experience, as a general matter. The higher the ROF number, the higher the temperatures the mortar tube will experience. For an 81 mm finless mortar tube of conventional construction, the maximum ROF is 25 rds/min for 1 minute and 5 rds/min sustained. For an 81 mm finless mortar tube made in accordance with the invention, the maximum ROF is 30 rds/min for 2 minutes and 15 rds/min sustained. Thus, the conventional tube has a very low ROF and is unable to satisfy future requirements for operational use.

FIG. 3 graphically shows temperature profile vs. axial position in the tube for a conventional 81 mm tube (lower curve) and the inventive 81 mm tube (upper curve) at each tube's maximum permissible ROF. The inventive tube's temperature is approximately 400° F. hotter, because of the ability to handle a larger ROF. The conventional mortar tube cannot handle an increased ROF, as needed to meet future requirements, without adding cooling fins.

The mortar tubes made according to the invention weigh approximately thirty percent less than conventional mortar tubes. After much experimentation, analysis and testing, it was discovered that, for a 60 mm superalloy mortar tube, the preferred wall thickness is in the range of about 1.75 mm to about 2.5 mm. For an 81 mm superalloy mortar tube, the preferred wall thickness is in the range of about 2 mm to about 5 mm. For a 120 mm superalloy mortar tube, the preferred wall thickness is in the range of about 2 mm to about 6.75 mm.

While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof. 

1. A mortar tube made of a superalloy.
 2. The mortar tube of claim 1 wherein the superalloy is one of cobalt based, iron based and nickel based.
 3. The mortar tube of claim 1 wherein an external surface of the mortar tube is free of cooling fins.
 4. The mortar tube of claim 1 further comprising an integral blast attenuation device.
 5. The mortar tube of claim 1 having an inside diameter of about 60 mm.
 6. The mortar tube of claim 5 having a wall thickness in a range of about 1.75 mm to about 2.5 mm.
 7. The mortar tube of claim 1 having an inside diameter of about 81 mm.
 8. The mortar tube of claim 7 having a wall thickness in a range of about 2 mm to about 5 mm.
 9. The mortar tube of claim 1 having an inside diameter of about 120 mm.
 10. The mortar tube of claim 9 having a wall thickness in a range of about 2 mm to about 6.75 mm.
 11. A method of making a mortar tube, comprising: providing a superalloy material; and forming the mortar tube from the superalloy material.
 12. The method of claim 111 wherein the forming step includes forming a finless mortar tube.
 13. The method of claim 11 wherein the forming step includes forming a blast attenuation device integral with the mortar tube.
 14. The method of claim 11 wherein the forming step includes machining the mortar tube from the superalloy material.
 15. The method of claim 11 wherein the forming step includes forming the mortar tube from the superalloy material using a metal flow-forming process.
 16. The method of claim 11 wherein the providing step includes providing a superalloy material selected from the group consisting of cobalt based, iron based and nickel based. 